Technological advances in digital transmission networks, digital storage media, very large scale integration devices, and digital processing of video and audio signals have been converging to make the transmission and storage of digital video economical in a wide variety of applications. Because the storage and transmission of digital video signals is central to many applications, and because an uncompressed representation of a video signal requires a large amount of storage, the use of digital video compression techniques is vital to this advancing art. In this regard, several international standards for the compression of digital video signals have emerged over the past decade, with more currently under development. These standards apply to algorithms for the transmission and storage of compressed digital video in a variety of applications, including: video-telephony and teleconferencing; high quality digital television transmission on coaxial and fiberoptic networks, as well as broadcast terrestrially and other direct broadcast satellites; and in interactive multimedia products on CD-ROM, Digital Audio Tape, and Winchester disk drives.
Several of these standards involve algorithms based on a common core of compression techniques, e.g., the CCITT (Consultative Committee on International Telegraphy and Telephony) Recommendation H.120, the CCITT Recommendation H.261, and the ISO/IEC MPEG-1 and MPEG-2 standards. The MPEG algorithms have been developed by the Moving Picture Experts Group (MPEG), part of a joint technical committee of the International Standards Organization (ISO) and the International Electrotechnical Commission (IEC). The MPEG committee has been developing standards for the multiplexed, compressed representation of video and associated audio signals.
The MPEG-2 standard describes an encoding method that results in substantial bandwidth reduction by a subjective lossy compression followed by a lossless compression. The encoded, compressed digital data is subsequently decompressed and decoded in an MPEG-2 compliant decoder. The MPEG-2 standard specifies a very high compression technique that achieves compression not achievable with intraframe coding alone, while preserving the random access advantages of pure intraframe coding. The combination of frequency domain intraframe encoding and interpolative/predictive interframe encoding of the MPEG-2 standard results in a balance between intraframe encoding and interframe encoding.
The MPEG-2 standard exploits temporal redundancy for motion compensated interpolative and predictive encoding. That is, an assumption is made that “locally” the current picture can be modeled as a translation of the picture at a previous and/or future time. “Locally” implies that the amplitude and direction of the displacement are not the same everywhere in the picture.
The MPEG-2 standard further specifies predictive and interpolative interframe encoding and frequency domain intraframe encoding. It has block-based motion compensation for the reduction of temporal redundancy and discrete cosine transform based compression for the reduction of spatial redundancy. Under MPEG-2, motion compensation is achieved by predictive coding, interpolative coding, and variable length coded motion vectors. The information relative to motion is based on a 16×16 array of pixels and is transmitted with the spatial information. It is compressed with variable length codes, such as Huffman codes.
The ISO MPEG-2 compression standard specifies only the syntax of bitstream and semantics of the decoding process. The choice of coding parameters and trade-offs in performance versus complexity are left to the encoder developers.
One aspect of the encoding process is compressing a digital video image into as small a bitstream as possible while still maintaining video detail and quality. The MPEG standard places limitations on the size of the bitstream, and requires that the encoder be able to perform the encoding process. Thus, simply optimizing the bit rate to maintain desired picture quality and detail can be difficult.
Preprocessing of digital video pictures can be advantageous to the digital video encoding process. Vertical filtering is one such preprocessing technique that can be used to soften input pictures to the encoder and thereby reduce noise. This results in better compression, without loss of quality.